When nuclear fuel assemblies must be stored, they are generally placed in storage racks, submerged in pools. These racks usually have a rigid structure or “skeleton” defining a plurality of cavities or “cells,” parallel to each other and oriented vertically when the rack is submerged in the pool. At least some of these cells receive a sleeve defining a cavity for receiving a nuclear fuel assembly. In order to reduce the distances between each cell as much as possible and thereby increase the storage capacity of the rack, while ensuring the sub-criticality of the latter part when it is loaded with nuclear fuel assemblies, this sleeve has a neutron absorption power, as described in document FR 2 759 484. To do this, plates made from a so-called “neutron absorbing” material, such as borated stainless steel, are used. Moreover, a water blade is present between the cell of the rigid structure and its associated sleeve, making it possible, together with the neutron absorbing material, to avoid any risk of criticality.
Traditionally, the cell, the sleeve and the nuclear fuel assembly have, in section, a same general shape, usually square, but more generally polygonal.
In document FR 2 759 484, the design of the sleeves is relatively simple, but can be problematic, given that the fuel assembly is in direct contact with the plates made from a neutron absorbing material. This situation results in a significant risk of rifling damage of the neutron-absorbing plates during operations for loading and unloading assemblies in the cavity.
Furthermore, the edge of the neutron-absorbing plates must be serrated so as to ensure their maintenance in position. In relation to plates with straight edges, these serrations require additional manufacturing and checking operations that do not make it possible to fully meet economic criteria.
In document EP 0 175 140 A1, sleeves are also inserted in cells. Nevertheless, the concept presented in that document has the drawback of using a neutron-absorbing material requiring its confinement between sheets, substantially complicating the production of the sleeves.
In document JP 05 040195 A (Toshiba Engineering Co), the sleeves are formed by openwork tubes, the neutron-absorbing plates being positioned in the openwork portions. The thickness of the walls forming the tubes must then be at least equal to that of the neutron-absorbing plates, which does not make it possible to optimize their thickness and therefore to comply with mass criteria. Furthermore, the tubes are made using costly machining operations so as to be able to receive the neutron-absorbing plates in the openwork portions. Lastly, the neutron-absorbing plates undergo discontinuities over a length at least equal to several times their thickness at elements keeping the profiles of the tube in position, which is detrimental to the performance of the sleeves from a criticality perspective by facilitating the neutron interaction between adjacent fuel assemblies.